Software and System Safety Research Group: A White Paper
Nancy Leveson
Aeronautics and Astronautics
Massachusetts Institute of Technology
leveson@mit.edu
Introduction
Computers are rapidly becoming an integral part of nearly every engineered
product, as well as controlling the manufacturing process for products:
Computers control consumer products, commercial aircraft, nuclear power
plants, medical devices, weapon systems, aerospace systems, automobiles,
public transportation systems, and so on. Virtually nothing is engineered
and manufactured in the U.S. today without
computers affecting the design, manufacturing and operation. Not only do
products use computers to operate better or cheaper---``smart'' automobiles
and appliances are examples---but complex systems are incorporating designs
that cannot be operated without computers---for example, unstable aircraft
and space vehicles that cannot be operated successfully by humans alone.
David Hughes wrote in a recent editorial in Aviation Week and Space
Technology:

``Information technology is becoming a key part of everything the aerospace
and defense industry does for a living, and as the century closes it is
computers and software that hold the keys to the future. The [aerospace]
industry is being transformed from dependence on traditional manufacturing
into something that looks more like IBM and Microsoft with wings.''
At the same time that computers are becoming indispensable in controlling
complex engineered systems, quality and confidence issues are increasing in
importance. We are hearing more and more about failures due to computers:
Software errors have resulted in loss of life, destruction of property,
failure of businesses, and environmental harm. Computers now have the
potential for destabilizing our financial system. Some large
government-financed projects are in trouble or have been canceled because
of difficulty in assuring the quality of the software.

One of the reasons for the problems is that these systems require that
standard engineering techniques be extended to deal with new levels of
complexity, new types of failure modes, and new types of problems arising
in the interactions between components. Computers exacerbate engineering
problems
by allowing levels of complexity and coupling with more integrated,
multi-loop control in systems containing large numbers of dynamically
interacting components. We are attempting to build systems where the
interactions between components cannot be thoroughly planned, understood,
anticipated, or guarded against. The fundamental problem is intellectual
unmanageability: Increased complexity and coupling make it difficult for
the designers to consider all the potential system states or for operators
to handle all normal and abnormal situations and disturbances safely and
effectively. The failures in these systems are arising in the interactions
between components. While we train engineers to be experts in individual
fields, these complex heterogeneous systems (composed of electromechanical,
digital, and human components) require knowledge and techniques that span engineering disciplines.

The Software and System Safety Research Group is a response to these problems.
It's goal is to act as a focus for interdisciplinary research, education, and
development to support the engineering and use of computers embedded in
and controlling complex engineered systems. This white paper discusses
the problem being attacked, attempts to delineate why the problems have
not already been solved, and suggests some specific research topics that
we feel are of critical importance in stretching the current limits of complex system engineering.

The Problem
During and after World War II, technology expanded rapidly, and engineers
were faced with designing and building more complex systems than had
previously been attempted. The creation of systems engineering as a
discipline received much of its impetus from aerospace programs, but
the new systems engineering techniques were soon adopted and applied
to the process industry (chemicals and nuclear power), transportation
systems, and other complex engineered systems.

As the systems we wanted to build became too complex or too time-critical to
be controlled by humans or even electromechanical devices, computers started
to be used to take over at least part and sometimes all of the control
functions. Not only are computers flexible and seemingly limitless in
their power, but they work at a speed that cannot be duplicated by any
other means and are relatively cheap besides. These characteristics allow
us to engineer products and complex systems that were previously
inconceivable. The computer has freed us from many of the physical limits
of electromechanical devices, but we are now faced with practical
limitations in our ability to engineer the software parts of these
systems.

As electromechanical controllers are replaced by computers, many of
the basic engineering and systems engineering techniques that were developed
to cope with complex systems are no longer adequate. Software adds the
potential for introducing a level of complexity not previously possible:
Most control software is too complex for complete mathematical analysis
and yet too structured for statistical analysis. At first, heroic human
effort, brute force techniques, and tremendous amounts of money were able
to get large software projects like the Space Shuttle control system
finished successfully. However, our ambitions are starting to outstretch
the limits of what brute force and money can accomplish, and the technology
to build such systems and to provide the needed confidence in their quality
does not exist.

As an example, the Space Shuttle software, one of the largest and most
ambitious software development projects of the 1970's, contains about 400,000
lines of code. NASA put enormous amounts of money into its development
and still spends approximately $100,000,000 a year to maintain it. In
contrast, even automobiles and some household products now have or will
soon have that much software in them. More complex projects, such as
upgrades to the U.S. Air Traffic Control System, Space Station Freedom, commercial and military aircraft, and even telephone switching systems
contain millions of lines of code. To build such software may require
hundreds and sometimes thousands of people, and just organizing these
projects is a massive undertaking. The result of not solving these
system and software engineering problems may be failures in our
attempts to build the complex systems of the future. As just one
example, the huge cost overruns and technical difficulties encountered
in building a new U.S. Air Traffic Control system led to cancelling large
parts of it a few years ago. The more recent scaled back attempts to
provide limited upgrades are also running into problems. The past six
months have seen the failure of five satellite launch attempts, several
of them blamed on software, including the most recent failure of a
Titan IV-B/Centaur Milstar mission that has been billed as the most
costly unmanned accident in the 50-year history of Cape Canaveral
launch operations.

Merely producing enormous amounts of code is not enough. The potential
for losses---human, environmental, and financial---with these
computer-controlled systems makes quality of paramount importance. Virtually
all non-trivial software has errors in it, and we do not currently have the
capability to locate and correct these errors. We are putting reliance on
human products that we cannot demonstrate are trustworthy, and it is getting
worse as the complexity of the systems we attempt to build increases.

While the U.S. has been ahead of the rest of the world in software engineering,
this situation is starting to change. The EEC countries and the Japanese
are catching up and may be ahead in achieving high quality levels. Currently,
the Japanese outstrip the U.S. in quality and productivity for relatively
simple software systems, and they are now working on the engineering of more
complex systems. The EEC countries have launched major initiatives in
software engineering, including applying mathematical techniques to software,
and are now ahead of the U.S. in this and other areas. The center of gravity
of software engineering research in general may now have shifted to Europe.

Why the Problems
Although major initiatives are currently missing, certainly a great deal
of effort has been and still is being applied to these problems. Why
are we still having trouble building embedded software?

One answer to this question is that we have made progress, but the problems
we are facing are increasing at a faster rate. The term ``software crisis''
to describe the problems of software engineering was introduced in the late
1960s and still is being used. However, this usage is misleading. Today
we have relatively few problems building the typical software systems of
the 1960s. Man's reach always outdistances his grasp---as we learn how
to build one type of software system successfully, we immediately want to accomplish more.

But we cannot blame all our limitations on increasing expectations. Although
a large number of researchers have been working on software engineering,
their results have had limited use in real systems. There may be several
reasons for this.

First, academic researchers have concentrated on the mathematical aspects
of problems and solutions while ignoring human factors and the necessarily
informal aspects of software development. While mathematical techniques are
useful in some parts of the process, informal techniques will always
be a large part (if not the majority) of any software development effort,
and, indeed, most engineering projects in general. Researchers often focus
exclusively on formal or on informal aspects of software development without considering their interaction.

Formalism is crucial in developing software for critical systems, but
the limits of modeling reality must be taken into account: (a) the actual
system has properties beyond the model, and (b) mathematical methods
cannot handle all aspects of system development. No comprehensive approach
to developing critical systems will, in the foreseeable future, be entirely
formal while informal approaches alone cannot provide adequate confidence.
Our approaches must be driven by the need to systematically and
realistically balance and integrate mathematical and nonmathematical
aspects of software development.

Often the result of research is methodologies that cannot be incorporated
into practice by developers and maintainers. Developing understanding about
how to build critical software is not enough. The methodologies must include
training and technology transfer and must be usable by those with typical
software engineering backgrounds. The methodologies must also incorporate
models that are closely related to the problem domain and the
way that application experts think about their problems, not necessarily
the way that researchers look at the problems.

One serious drawback of past and current software engineering research
is lack of scalability. Researchers have developed techniques that work only on small systems. Mathematical techniques have, for
the most part, been used only on very limited properties and on unrealistically small problems. Most any analysis technique works on
a toy problem. There is reason to believe that software development in
the large is so different than the toy problems found in most research
papers that many published techniques may not apply to real projects.
We need to find a balance of formal and informal techniques that scale
by considering, from the start, problems of realistic size and complexity.
Software engineering researchers rarely validate their techniques and
theories on realistic software. Given the complexity of the systems we
are attempting to build, the only convincing argument that an
approach will work in practice is to validate techniques on real systems.

Successfully building software for complex systems demands that qualities
such as reliability, safety, security, and timing be rigorously addressed
and systematically built into the software from the beginning. In addition,
simply concentrating on initial development is not enough: These qualities
must be preserved as the software evolves during its lifetime. Independent
efforts to ensure individual qualities in narrow domains, e.g., security,
have made significant progress. However, no approach exists that combines
diverse techniques into an integrated methodology for developing and
maintaining software for critical systems. Furthermore, the methodologies
that are developed must be usable by other than their developers and must
be able to be incorporated into practice by software developers.

Specific Areas for Research
We believe the following areas are of special importance and difficulty
in engineering complex, computer-controlled systems and thus are appropriate
avenues of research. Many of these research goals are at the interface
of what has typically been considered software engineering concerns and
those of system engineering.

Modeling and Analysis
Whereas in the past engineers were able to reuse standard designs that
had been perfected over many years, most of the new systems using computer
control require new designs. The complexity of these systems, furthermore,
does not usually allow us to build physical prototypes and experiment with
them enough to learn how to improve our designs. Instead, mathematical
models must be used to verify certain required properties. An important
research topic involves defining powerful and efficient modeling languages
and analysis techniques to allow prediction and accumulation of information
that will aid in the system and software design and verification process.
Although many modeling techniques have been proposed, most consider only
very limited system aspects and do not adequately handle such things as
timing, failures, and hazards.

Analysis is an intrinsic part of any engineering discipline---no bridge
or space vehicle is constructed without enormous amounts of modeling,
calculation, checking, and revision. Today's software engineer simply
lacks the theory to bring to bear on engineering problems. Gerhart has
suggested that the scientific basis that currently exists is a collection
of micro-theories , each reasonably well understood but isolated by
its own notation, techniques, and world view. Most models are related to
single qualities, such as security or reliability. A few general models
exist with extensive theories, such as Petri nets, but these models often
lack the power to provide the required information to designers or to
address the variety of qualities required in large and complex systems.
Most models also provide little help in comparing alternative system designs.

Not only do we need better formal methods, but we need ways to interface
them to human abilities and to informal methods. The techniques and tools
we develop must be usable by software developers and not just by the
researchers that developed them, and they must be integratable into
normal software development environments.

Engineering for Quality
One of the most important issues in complex systems is achieving and
assuring quality---identifying and resolving tradeoffs between various
qualities, determining how to achieve multiple qualities, and providing
confidence or assurance that particular systems will exhibit required
qualities over their lifetime. Currently we have no way to achieve or
assure high levels of software quality.

Essential system-wide properties (reliability, safety, security,
and modifiability) must be built in from the beginning; they
cannot be added on or simply measured afterward. Up-front planning
and changes to the development process are needed to achieve particular
objectives. These changes include using notations and techniques for
reasoning about system properties, constructing the system to achieve particular properties, and validating (at each step so that it is done
early) that the evolving system has the desired properties. Central
to this problem is the consideration of the interactions among critical
system properties and potential conflicts among them. Research about
different kinds of properties are usually associated with distinct,
often insular, groups.

An unwarranted assumption is often made that independent approaches to
achieving specific software and system qualities can be easily composed.
Unfortunately, this is not true. As just one example, approaches to ensure
usability or reliability properties may (and often do) interact in
important but indirect ways with approaches to ensure safety properties.
Many techniques can be found to attack particular subproblems, but these
techniques may not be easily integrated or may be too costly if very
different procedures are required for each critical property or if each
part of the software development process does not build on the results
obtained in the previous steps. We need integrated
methodologies for developing and maintaining software that encompass the
entire development process and consider multiple and perhaps conflicting
goals.

Providing Assurance
More than half of software development effort goes into confidence building
activities (verification and validation). We are able to execute and test
only a small fraction of the possible system states before software is
put into operational use. Yet, particularly for critical systems, high
confidence is often a prerequisite for certification or use.

While dynamic analysis, i.e., testing, will always have an important place
in providing confidence, cost and criticality are increasing the need
for static analysis of software that can provide assurance over the entire
range of software states. Testing and analysis should and can support
each other, with testing providing confidence in the correctness of the
assumptions made in static analysis. We need to provide more affordable
and effective testing while at the same time exploring the potential for
static analysis of important properties and understanding the interaction
between these two approaches to assurance.

Human-Computer Interaction
Most complex systems require a combination of human and computer control,
where humans provide intelligence and problem-solving ability while
computers handle aspects requiring speed and computational power. Challenges
exist in determining how to allocate tasks between humans and computers and
how to design the features of this interaction so that the unique
capabilities of each are optimized. Simply replacing the human by
computers, the obvious and often only approach considered, may not result
in the most efficient, useful, and safe systems. The desired end is a
partnership between the computer and the human that is superior to either
of them working alone.
Serious accidents are starting to occur in aircraft and other shared
control systems where the design of the interaction between computers
and humans is being blamed rather than failures or errors on the part of
either of these system components. Although much research exists on how
to make usable and ``friendly'' computer interfaces, very little exists
on how to integrate computers and humans in a complex system.

In a slightly different context, a better understanding also is needed of
the way to design software engineering tools and languages in order to
minimize the number of errors that are introduced during software
development and to provide usable and useful tools to software
developers. One of the roadblocks in making progress on these problems
is the lack of scientifically established information upon which to make
decisions about the design of software engineering tools and techniques.
There has been a great deal of study of the mathematical and engineering
foundations of software engineering, but much less of the psychological
foundations. We need to establish these foundations.

Evolution
Software engineering approaches often concentrate on initial software
development and not on the continual evolution of the software and its
environment. Software is continually changing and evolving, not only
because of the discovery of latent errors, but primarily because of
changes in the operating environment, in the needs of the end users, and
in the underlying technology. We believe that software must be designed
to be changeable without compromising the confidence in the properties
that were initially verified. Sometimes decisions will have to be made
not to change critical software if the risk is unwarranted. We need
ways to make those decisions, ways to design and construct software so
that it can evolve over time without compromising critical properties,
and techniques to aid in the evolution and change process itself.

Risk Assurance and Assessment
Computers currently are being introduced into the control systems of
dangerous processes (such as nuclear power, public transportation, and
weapons) without any way to determine whether the associated risk is
reduced, the same, or increased. Because analog and mechanical
control systems with measurable risk are being replaced by computers,
we need to develop procedures that provide the same level of assurance
of acceptable risk.

Numerical risk assessments of physical systems usually are derived from
(1) historical information about the reliability of individual components
and models that define the connections between these components or
(2) historical accident data about similar systems. Neither of these
assessment approaches apply to software: Historical information is not
available, software is usually specially constructed for each use, and
random wearout failures are not the problem. Devising probabilistic
models of software reliability is an important research topic; they are
potentially very useful in software development. But their usefulness
in certifying safety is less clear.
The very low failure probabilities and high confidence in these
assessments that is required in safety-critical systems require more experience with the software than could possibly be obtained in any
realistic development process. More important, these models are measuring the wrong thing. Software reliability is defined as compliance
with the requirements specification, but accidents most often occur
as a result of flawed specifications, i.e., faulty assumptions about
the behavior of the environment or the required behavior of the software.
Software reliability prediction models assume that it is possible to
predict accurately the usage environment of the software and to anticipate
and specify correctly the appropriate behavior of the software under
all possible circumstances. Both of these goals are impossible to achieve.

Just as probabilistic evaluation may not be the most appropriate way to
provide confidence in the proof of a theorem, it may also not be the best
way to achieve confidence that software will {\em always} do the correct
or safest thing under {\em all} circumstances. An emphasis on formal
and informal verification, analysis, and review may be more appropriate
in evaluating a software and system design. We need more research on
procedures to identify software-related hazards, to eliminate and control
these hazards through design, to apply safety-analysis techniques during
software development to provide confidence in the safety of software
and to aid in the design of hazard protection, and to evaluate the
effectiveness of the analysis and design procedures to assess the level
of confidence they merit.

Qualitative risk assessment and assurance techniques need to be developed
if government and society are going to continue to allow the use of
computers to control processes that potentially affect public safety.

Summary
Industry and government are currently struggling with building complex,
computer-controlled systems, and often unsuccessfully as witnessed by
failures of major projects. We envision the MIT Center for Software
Research as a place where academia, industry, and government can come
together to focus on stretching the limits of the complexity of the
systems we can successfully engineer.

from : http://sunnyday.mit.edu/white-paper.html
- posted by sankarshan @ 2/12/2003 06:39:00 AM 0 comments